Issue

Enabling Next-generation Stacked-die Applications

In today’s packaging industry, traditional single-die designs are being replaced by multi-die configurations that offer increased functionality and performance at a lower cost. According to recent research from SEMI and TechSearch International, stacked-die packages are expected to grow at an average annual growth rate of 12% from 2008 to 2010.1 The potential efficiencies of these package technologies are undeniable, but the manufacturing challenges — particularly in relation to handling and materials considerations — loom large. Current techniques using traditional die-attach pastes do not offer the mechanical support, throughput, and yield advantages that modern die-attach films can provide to ultra-thin wafers.

Die-attach film (DAF) technologies in the form of dicing die-attach films (DDF) and flow-over-wire (FOW) materials have emerged as efficient products for next-generation die stacking applications.

Conventional Die-attach Paste Process

Regardless of the die attach material used – film or paste – the wafer must first be mounted onto dicing tape to secure it during the dicing procedure. Once the wafer has been diced, individual die move on to the placement process. Die stacking using die-attach paste requires more time and equipment than stacking procedures that use DAF. Die-attach paste is dispensed onto the first die in the stack (D1), the second level die (D2) is placed and then the 2-die stack is transferred into the cure oven. This procedure must be repeated for each die in the stack. So, a four-die-stack package would see four dispensing steps and four cure oven trips. After the entire stack is complete and all layers have been cured, the package then moves to wire bonding (Figure 1).

The ultra-thin die used in modern stacked package applications are difficult to handle, and once removed from the dicing tape for placement, loses any mechanical support provided by the film. So, during the placement process, the thin die may crack and/or curl from the handling stresses induced by the standard process.

Dicing Die-attach Film

DDF, which combines both dicing tape and die-attach material into one product, is proving to be a good replacement for die-attach paste. From a performance point of view, DDF offers control of paste bleed, creeping effect to die edge, as well as consistent bondline control (BLT) at the desired thickness. Though earlier generation film materials did not offer the placement speeds delivered by that of die attach pastes, newer DDF formulations enable die placement times that are comparable, if not faster, than die-attach pastes. On average, placement speeds using die-attach pastes are 0.3 seconds. Today’s DDF technology delivers wetting ability with placement times of as little as 0.1 seconds, challenging the throughput argument often used by proponents of die-attach paste.

The most important advantage of DDF materials, however, is the support films offer for thin wafer handling, which allows even thinner die to be used for die-stacking applications. Package die stacks using 75 μm have been achieved, and even 25-μm wafer package die stack is now possible with new die attach film technologies.

When using DDF, the wafer is laminated onto the film then diced, and the die is picked and placed on the stack. Advanced DDF materials do not require any cure step prior to wire bonding – each die in the stack can be picked and placed and then the entire package moves to wire bonding. There are some DDFs that require a UV process to release the dicing tape adhesive from the die-attach, which may add time and process control challenges to the die stacking procedure. Pressure-sensitive DDF materials do away with UV requirements, thus improving throughput and reducing costs by eliminating the need for UV equipment.

Figure 3. Using film for support has clear wafer handling and processing advantages.

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With the performance benefits and critical handling advantages afforded by these materials (Figure 3) DDF is undoubtedly one of the more cost-effective key enablers for modern stacked die packages.

FOW Film

A recent development in die-attach film technology is FOW materials that allow same-size die stacking. FOW film is formulated to incorporate die-attach film and dicing tape into one product. Because these materials are designed to flow over the wire bonds of the die below the one being placed, the need for spacer die or sequential die staggering is eliminated. For no-cure FOW materials, the process is identical to that of DDF. The wafer is laminated onto FOW film, diced, and then the die are picked and placed. The stack moves to wire bonding and then on to molding. While no-cure FOW products are ideal for maximizing UPH, some manufacturers are still more comfortable incorporating a cure step into their die stacking process — particularly for same-size die stacks — as a precautionary step to prevent any die movement during molding. Consequently, two alternative cure materials have been designed to meet the varying requirements of certain applications. The first, a quick-cure FOW (FOW A) offers the ability to cure inline on the wire bonder with no separate oven cure step required, as it can cure within 10 minutes at 175??C. The second material, FOW B, incorporates a cure system that has enough strength to withstand wire-bonding after being partially cured at 165 - 175??C for 10 minutes. Once all die have been stacked and wire bonded, the package travels to a cure oven, where full curing can take place in a little as 30 minutes at 165??C. With either the FOW A or FOW B material, it is essential that a low-voiding formulation is selected, because once the die is placed, the die attach must be void-free or voids will remain after cure, which is not an acceptable condition for some applications. Initial testing even suggests that perhaps a separate cure step is not necessary, and that in both cases the material would cure enough during wire bonding step for the die stack to withstand the molding process, but more work is being done.

With any of these FOW film types, certain materials characteristics should be evaluated. Because the FOW encapsulates wire bonds of the stacked-die package, it is essential that the FOW is ionically stable to avoid corrosion issues that could arise from humidity exposure. The die sizes used in these packages are large, so stress is a major concern, as it is affected by the modulus, Tg, and CTE of the film materials. Additionally, the visco-elastic properties of the FOW materials must be optimized so that the film can be placed over the wire bonds without any wire-bond deformation (Figure 4).

Conclusion

New die-attach film technologies in the form of DDF and FOW materials are enabling stacked-die applications to move into mainstream manufacturing. Device footprints can now become even smaller, thinner and higher functioning at a lower cost and faster processing times. While further advances in film technology are currently underway, today’s materials give packaging specialists the performance and processing capabilities needed to manufacture advanced, stacked-die applications.